Joint sockets to be used as counterbearings or support bearings for the synthetic femur head are preferably made of highly polymerized polyethylene. These synthetic sockets are often implanted directly into the acetabulum which has been ground down accordingly. But based on leading opinion today, a bone-metal-polyethylene configuration is preferred over the bone-polyethylene configuration, as presented, for example, by Mark B. Coventry in the Year Book of Orthopedics, Chicago, Year Book Medical Publishers Inc., 1985, p. 175.
In order to realize the synthetic hip socket in the bone-metal-polyethylene configuration preferred today, a threaded metal ring is known, e.g. from European patent application No. 0142759 which is inserted into the acetabulum and into which a polyethylene socket is clamped.
Polyethylene hip joint sockets are also known whose rim is partially extended i.e. has a partially raised shoulder, in order to reduce the danger of slippage of the synthetic femur head from the socket, that is to reduce the danger of dislocation. Such joint sockets, e.g., are known as "poly-dial inserts" of the Joint Medical Products Corporation, advertisement in "The Journal of Bone and Joint Surgery", 68-A, April 1986, p. 91, or of polyethylene inserts of the Waldemar Link GmbH & Co., advertisement in "The Journal of Bone and Joint Surgery", British Volume 680B, May 1986, following p ii.
In order to be able to install a socket prosthesis even under unfavorable circumstances with the pelvis, such as in the case of bone defects, a larger support surface has to be created between the metal ring and the pelvis. For this purpose, German patent No. 2314708 calls for a metal shell with a support flange which is anchored in the acetabulum with bone cement and in which the polyethylene socket is inserted, similarly with the help of bone cement.
In order to achieve reliable power transfer from the prosthesis to the pelvis despite reduced or weakened bone substance, socket cover shells made of metal, as presented by M. E. Mueller, or socket support shells, as presented by H. B. Burch and R. Schneider, are used. See, e.g., R. Schneider, "Total Prosthesis of the Hip", Bern, Hans Huber Publishers, 1982, p. 30 ff. The socket cover shell based on M. E. Mueller and the socket support shell based on H. B. Burch and R. Schneider are also known from the entry as the Swiss sample and model Nos. 110.826 and 110.827, respectively.
These socket cover shells or socket support shells are anchored in the bone, primarily with screws. The polyethylene socket is secured in the metal shell by means of bone cement. As a result, the bone cement also penetrates the holes of the metal shell and contributes to the mounting of the metal shell in the bone.
In order to secure the acetabulum part of a hip prosthesis in the pelvis without using bone cement, P. Schuster in Swiss patent application No. 3252/84-9 proposed a metal shell with a pole area that is essentially parallel to the equator of the sphere and which has knife-like edges that are distributed evenly across the periphery of the sphere. The external shape has the form of an octagon at the equator level such that there is the possibility for primary fixation in the pelvic bone by means of screws.